This invention relates to a multilayer wiring structure featuring use of a polyimide for the insulating layer. More particularly, the invention pertains to a multilayer wiring structure which is reduced in thermal stress by use of a low-thermal-expansivity polyimide for the layer insulating films and also has high bonding reliability between conductor/insulating film or insulating film/conductor or at the interface between the insulating films.
Polyimide resins are used as material of layer insulating films for multilayer wiring boards as said resins are excellent in heat resistance, chemical resistance and insulating properties and low in dielectric constant. However, the conventional polyimides, when used for forming the layer insulating films, present the problem of cracking or imperfect adhesion due to build-up of thermal stress after heat curing, that is, the stress caused by the difference in thermal expansion coefficient between the substrate or wiring material and the polyimide. For avoiding this problem, it is desirable to use a polyimide having a thermal expansion coefficient as close to that of the substrate or wiring material as possible. For instance, it is effective to apply, for forming the insulating films, a low-thermal-expansivity polyimide having a thermal expansion coefficient of 2.times.10.sup.-5 K.sup.-1 or less, which is close to that of the substrate or wiring material, owing to its rod-like structure as disclosed in Japanese Patent Application Kokai (Laid-Open) No. 61-176196. However, when a film or layer of a material is formed directly (without applying any treatment) on an insulating layer using a perfectly cured low-thermal-expansivity polyimide, there could take place exfoliation at the interface between the insulating layers or between the insulating layer and wiring metal, and thus there arises the problem of reliability. As a method for solving such a problem, Japanese Patent Application Kokai No. H1-245586 discloses use of a flexible printed board in which the insulator consists of a layer of a high-thermal-expansivity resin contacting the conductor and a layer of a low-thermal-expansivity resin contacting the first-said resin layer for improving adhesiveness between the conductor and the insulator. However, in the multilayer wiring boards having multilayer wiring comprising a plural number of insulating film layers and a plural number of conductor patterns, such as wiring boards for modules, multilayer flexible printed boards, LSI multilayer wiring boards, etc., high adhesiveness is required not only for conductor/polyimide interface but also for substrate/polyimide and polyimide/polyimide interfaces, so that it was necessary to take a measure that can satisfy both requirements for low-thermal-expansivity and high adhesiveness at various interfaces.
As a method for laminating a polyimide with low adhesiveness, use of FEP (perfluoroethylene-perfluoropropylene copolymer) as a heat-fusible adhesive is proposed in Japanese Patent Application Kokai No. 57-83432. However, FEP is lower in heat resistance (especially in softening point and melting point) than polyimides and can not provide satisfactory heat resistance.
Also, the idea of laminating the cured polyimide films by using a half-cured polyamic acid film is proposed in Japanese Patent Publication No. 57-5229, but little improvement of adhesiveness is admitted with the perfectly cured low-thermal-expansivity polyimide.
It is known that silicon-modified polyimides such as disclosed in Japanese Patent Application Kokai No. 62-227963 are enhanced in adhesiveness to glass or silicon substrates. However, in case a low-thermal-expansivity polyimide film is formed on a cured silicon-modified low-thermal-expansivity polyimide film, there is the problem that adhesion between the polyimide insulating films is low.
Thus, in forming the multilayer wiring boards of the type in which there exists various types of interfaces, it was impossible with the conventional techniques to satisfy both requirements for high adhesion at all interfaces and low thermal expansivity at the same time.
As described above, it was difficult to form a multilayer wiring structure by using a low-thermal-expansivity polyimide for the layer insulating films.